The present invention relates to an electrostatic chuck for holding and heating a substrate in a process chamber.
In the fabrication of integrated circuits, an electrostatic chuck is used to hold a substrate in a chamber to prevent movement or misalignment of the substrate during processing. A typical electrostatic chuck comprises an electrode covered by a ceramic or polymer insulator. The substrate held on the chuck is processed by a plasma of process gas and the plasma heats up the substrate. Generally, a heat transfer gas is provided between the surface of the chuck and the backside of the substrate to enhance heat transfer rates therebetween and control temperatures across the substrate. For example, in the etching of electrical interconnects which have small dimensions and are positioned very close together, there is a need for precise control of the temperature of the substrate to provide reproducible and accurate shapes of the etched features. The heat transfer gas increases heat conduction between the chuck and the substrate and maintains more uniform temperatures across the surface of the substrate by either heating or cooling the substrate.
In addition to precise temperature control, it is also desirable to have a chuck capable of heating the substrate to temperatures higher than those provided by the heat load of the plasma. Faster integrated circuits are formed by making the electrical interconnects of the circuits out of materials having high electrical conductivities. For example, copper or platinum can be substituted for lower conductivity aluminum to make faster interconnects. However, these materials are often processed at higher temperatures; for example, etching of these materials can require heating the substrate to temperatures of 250 to 400.degree. C. Increasing the power level of the plasma can heat the substrate to higher temperatures by imparting more kinetic energy to the ions that strike the substrate. However, it is not always desirable to increase the energy of the plasma ions because a higher energy plasma can cause charge up damage of the layers below the etched layer or otherwise change the shape of the etched features.
The substrate can also be heated by applying heat to the substrate from an external heat source, such as an infrared lamp outside the chamber, that projects radiation through a window in the chamber. However, infrared lamps are difficult to use in chambers that do not have little space for a window, for example, when inductor coils or other processing components are adjacent to the chamber walls. Furthermore, the inefficient transmission of heat through the volume of the chamber results in a long response time to raise the substrate to the desired temperature. This makes it even more difficult to maintain the temperature of the substrate within a narrow temperature range, especially when the heat load of the plasma in the chamber fluctuates during processing.
In another approach, described in U.S. Pat. No. 5,280,156, the electrostatic chuck comprises an electrode and heater, both of which are embedded in a monolithic ceramic of aluminum nitride. However, it is difficult to form a gas-tight seal between the hard ceramic and the overlying substrate to hold the heat transfer gas below the substrate. Leakage of heat transfer gas causes portions of the substrate near the leakage points to become hotter or colder than other portions of the substrate. This results in non-uniform temperatures and uneven processing rates across the substrate. In addition, the ceramic insulator comprises small grains that separate and flake off when their grain boundary regions are eroded by the corrosive process gas to form particles that contaminant the substrate. Also, different thermal expansion coefficients of the heating element and ceramic can lead to thermal stresses that result in failure of the chuck and loss of the entire substrate.
Therefore, there is a need for an electrostatic chuck that can securely hold a substrate and quickly heat the substrate during processing. It is further desirable to have an electrostatic chuck that can rapidly raise or lower the temperature of the substrate to regulate its temperature regardless of fluctuations in the plasma heat load. It is also desirable for the chuck to exhibit little or no erosion or particle generation during processing.